So it can be easy to miss something right in front of our faces: We’re living in an era when blacks have essentially played kingmaker in the most important elections in the nation (thus also the world). And it’s happening again, as Bernie Sanders and Hillary Clinton battle to win the South Carolina primary on Feb. 27.

Mr. Sanders’s first move after his resounding victory in New Hampshire last week was to travel to meet Al Sharpton at Sylvia’s Restaurant in Harlem the next morning. He’s under no illusion: If he can make inroads with black voters and other nonwhite voters in the remaining primaries, he’ll have a chance against Mrs. Clinton. If he can’t make those gains, he won’t, as my colleague Nate Cohn has calculated. Recent history makes it clear:

■ In the 2008 Democratic primaries, Mr. Obama could not have won without unified black support. Mrs. Clinton was the clear favorite. In late 2007, a Pew poll showed her ahead of Mr. Obama by 14 points in South Carolina.

But after Mr. Obama won Iowa, black voters swung behind him, and he defeated her in South Carolina by 29 points. According to exit polls, blacks made up 55 percent of the South Carolina Democratic primary electorate, and Mr. Obama won 78 percent of that vote to Mrs. Clinton’s 19 percent. He went on to roll in states with large black populations, and he needed every bit of that help, barely holding off Mrs. Clinton for the nomination.

■ In the 2012 presidential election, President Obama outdid Mitt Romney by 3.9 percentage points. There is not just one reason that Mr. Obama won. According to exit polls, he drew the support of 73 percent of Asian-Americans and 71 percent of Hispanics. But the black voting bloc was even more one-sided, at 93 percent. And black turnout far surpassed that of Asian-Americans and Hispanics. According to Amy Walter and David Wasserman at the Cook Political Report, the percentage of blacks who voted (66.2) also bested the percentage of whites (64.1) — for the first time.

David Frum of The Atlantic noted that “Obama’s support dropped by 3.6 million votes between his election and his re-election, and “absenteeism was most marked among younger voters and Latinos,” but the participation of African-Americans “actually increased between 2008 and 2012.”

Being part of a monolithic voting bloc has downsides. If votes from African-Americans are not up for grabs, the Democratic Party can more easily ignore their concerns. If they were free agents, they would be in a better bargaining position. Eugene Robinson of The Washington Post once wrote: “In politics, as in business, competition is good. Monopolies inevitably take their customers for granted.”

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The Rev. Al Sharpton and Senator Bernie Sanders met at Sylvia’s Restaurant in Harlem the day after Mr. Sanders’s victory in the New Hampshire primary.CreditSam Hodgson for The New York Times

But in close races at the presidential level, leveraging almost the entire weight of a motivated voting bloc — blacks are expected to make up around 20 percent of the Democratic 2016 primary electorate — can be very powerful.

It would be easy to say that the only reason for that leverage and motivation is African-American support for an African-American candidate who became president. But the fierce bidding war for black voters in this year’s Democratic primaries, without a black candidate, suggests staying power.

To remain as power brokers, converting a nomination victory into a place in the White House is obviously a crucial step, but the party’s diverse coalition keeps growing and offering promise of future presidential victories. It’s very early, but PredictWise gives a generic Democratic nomination winner a 61-39 edge over a Republican winner in November.

For followers of political history, it’s a rich twist that the Southern strategy, which helped Republicans hold the White House for 20 out of 24 years starting in 1968, has given way to such influence by African-Americans. It all starts in South Carolina this cycle, with two especially important subsets of the black vote there.

An Upshot reader, Ed of Old Field, N.Y., wrote: “As white evangelicals are for Republicans, black evangelicals are an important constituency for Democrats, and Sanders in South Carolina will have as much of an uphill climb against their skepticism of him personally as Trump, for different reasons, had in Iowa. Not at all insurmountable, but a high hurdle.”

■ In the last South Carolina Democratic primary, black women made up 61 percent of the black vote. In the 2012 presidential election, black women voted at the highest rate of any group across race, gender and ethnicity, and 96 percent of them voted for President Obama, according to exit polls. It is not an exaggeration to say that black women, in formation and flexing their political power, could have the final say over whether Mrs. Clinton becomes the first female presidential nominee of either party.